Distributed antenna system for mimo communications

ABSTRACT

A method and apparatus for determining placement of a plurality of antennas of a distributed antenna system for handling MIMO signals includes, at a first location, simulating the communication of a first MIMO signal by a first remote unit over an air interface in an environment and, at a second location, simulating the communication of a second MIMO signal by a second remote unit over an air interface in the environment. The first and second locations are arranged within the environment to provide overlapping signal coverage of both the first MIMO signal and the second MIMO signal at a third location in the environment. Analysis is made of at least an imbalance of received power between the first and second MIMO signals within the environment at a third location in order to determine whether a desired capacity for MIMO communications with the system has been achieved at the third location.

RELATED APPLICATIONS

This application is a Continuation application of U.S. patentapplication Ser. No. 14/291,321, filed May 30, 2014, entitled“DISTRIBUTED ANTENNA SYSTEM FOR MIMO COMMUNICATIONS”, which is aContinuation application of U.S. patent application Ser. No. 13/025,697,filed Feb. 11, 2011, entitled “DISTRIBUTED ANTENNA SYSTEM FOR MIMOCOMMUNICATIONS”, which U.S. application claims priority to ItalianApplication No. BO2010A000077, filed Feb. 12, 2010, entitled“DISTRIBUTED ANTENNA SYSTEM FOR MIMO SIGNALS”, and is a Continuationapplication of U.S. PCT Application No. PCT/US2011/023991, filed Feb. 8,2011, entitled “DISTRIBUTED ANTENNA SYSTEM FOR MIMO COMMUNICATIONS”,which applications are all incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

Embodiments of the invention are directed to wireless communicationsystems, and specifically directed to a distributed antenna system forwireless MIMO communications.

BACKGROUND OF THE INVENTION

A contemporary wireless communication system, such as distributedantenna system, includes a plurality of remote units distributedthroughout a service area (e.g., a building) to provide coverage withinthe service area of the system. In particular, each remote antenna unitis typically coupled to a master unit, which, in turn, is coupled to atleast one single-input- and single-output (“SISO”) base transceiverstation (“BTS,” or more simply, “base station”).

Each remote unit generally transceives wireless signals with a number ofwireless devices, such as a telephone devices or computing devices inthe proximity of the remote unit. In particular, the wireless signalsfrom each remote unit are associated with one or more BTSs. Thus, thewireless devices may communicate with the system BTS's through any ofthe wireless signals from the remote units.

To improve such wireless communications, Multiple-Input/Multiple-Output(“MIMO”) technology might be utilized to provide advanced solutions forperformance enhancement and capacity in broadband wireless communicationsystems. It has been shown that substantial improvements may be realizedutilizing a MIMO technique with respect to the traditional SISO systems.MIMO systems have capabilities that allow them to fully exploit themulti-path richness of a wireless channel. This is in contrast withtraditional techniques that try to counteract multi-path effects ratherthan embrace them. MIMO systems generally rely upon multi-elementantennas at both of the ends of the communication links, such as at thebase station and also in the mobile device. In addition to desirablebeam-forming and diversity characteristics, MIMO systems also mayprovide multiplexing gain, which allows multi data streams to betransmitted over spatially-independent parallel sub-channels. This maylead to a significant increase either in the system capacity or in thedata throughput to each wireless device. Generally, distributed antennasystems cannot take advantage of MIMO technology because they are justdesigned to provide SISO wireless coverage.

For example, in traditional distributed systems, a wireless devicecommunicates with only one of the remote units, the signals of which aretypically isolated from signals of other remote units using base stationsectorization techniques. In this manner, the signals from differentremote units avoid interference due to overlap of coverage areas. Thewireless signals from each remote unit are typically at the samefrequency and carry the same data.

Additional problems occur with a distributed antenna system disposedwithin an indoor environment. For example, indoor environments are oftenassociated with increased amounts of multipath richness. Generally,internal building components (e.g., columns, pipes, walls, doors) aswell as objects inside that building (e.g., computers, desks, fixtures)cause an increasing of the scattering phenomena. Also for example, SISOdistributed antenna systems are typically designed to provide wirelesscoverage within a particular indoor environment. However, because of themultipath richness, antenna shadowing can occur depending upon theparticular layout, user position, and obstacles within that indoorenvironment.

Accordingly, it is desirable to improve upon existing distributedantenna systems taking advantage of MIMO technology in distributedwireless environments which may benefit such propagation conditions.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a method of deploying a distributedantenna system. The method comprises outputting at least a first signaland a second signal from a multiple-input and multiple-output (MIMO)base station and coupling first and second master units to the MIMO basestation, the first and second master units configured to receive thefirst and second signal, respectively. The method further comprisescoupling a first remote unit to the first master unit, the first remoteunit communicating the first signal a first air interface located withinan environment at a first location and coupling a second remote unit tothe second master unit, the second remote unit communicating the secondsignal over a second air interface within the environment at a secondlocation. The method then comprises analyzing at least an imbalance ofreceived power between the first and second signals determined withinthe environment at a third location to determine whether a predeterminedcapacity for MIMO communications with the system has been achieved.

Alternative embodiments of the invention also provide a method fordetermining the placement of a plurality of antennas of a distributedantenna system with a computing system of the type that includes one ormore processors and a memory. In those alternative embodiments, themethod comprises simulating a first remote unit communicating a firstsignal over a first air interface located within an environment at afirst location and simulating a second remote unit communicating asecond signal over a second interface located within an environment at asecond location. The method further comprises analyzing at least asimulated imbalance of received power between the first and secondsignals determined within the environment at a third location todetermine whether a predetermined capacity for MIMO communications withthe system has been achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a distributed antenna system consistentwith embodiments of the invention.

FIG. 2 is a detailed block diagram of a master unit utilized inembodiments of the invention.

FIGS. 3A and 3B are a detailed block diagram of a portion of a remoteunit utilized in embodiments of the invention.

FIG. 4 is a detailed block diagram of an alternate portion of a remoteunit utilized in embodiments of the invention.

FIG. 5 is a data plot illustrating the channel capacity increase as afunction of SNIR, and that further illustrates that, for a givenenvironment, the optimization of MIMO channel capacity depends upon theoptimization of the SNIR and the CCN consistent with embodiments of theinvention.

FIG. 6 is a data plot illustrating the typical CCN distribution in a“dense” indoor environment with co-polarized antennas consistent withembodiments of the invention.

FIG. 7 is a data plot illustrating the typical CCN distribution in an“open” indoor environment with co-polarized antennas consistent withembodiments of the invention.

FIG. 8 is a data plot illustrating the typical CCN distribution in a“large” indoor environment with co-polarized antennas consistent withembodiments of the invention.

FIG. 9 is an illustration of an indoor environment in which four remoteunits have been distributed and in which various imbalances between twosignals of a 2×2 MIMO scheme have been determined consistent withembodiments of the invention.

FIG. 10 is a data plot illustrating the effect of an imbalance of thereceived power of two signals on the CCN when that imbalance is fromabout 5 dB to about 10 dB.

FIG. 11 is a data plot illustrating the effect of an imbalance of thereceived power of two signals on the CCN when that imbalance is fromabout 5 dB to about 10 dB.

FIG. 12 is a data plot illustrating the effect of an imbalance of thereceived power of two signals on the CCN when that imbalance is fromabout 10 dB to about 15 dB.

FIG. 13 is a data plot illustrating the effect of an imbalance of thereceived power of two signals on the CCN when that imbalance is greaterthan about 15 dB.

FIG. 14 is a data plot illustrating the effect of the CCN and SNIR onthe capacity of a MIMO system consistent with embodiments of theinvention.

FIG. 15 is a flowchart illustrating a sequence of operations toselectively determine a location to deploy a plurality of remote units,or a plurality of antennas of remote units, to optimize the capacity ofa MIMO DAS in an environment consistent with embodiments of theinvention.

FIG. 16 is a flowchart illustrating a sequence of operations for a userto selectively tune the operation of a MIMO DAS in either a SU-MIMO modeof operation or a MU-MIMO mode of operation based upon a power imbalanceconsistent with embodiments of the invention.

FIG. 17 is a diagrammatic illustration of a MIMO DAS that includesco-located remote antennas within an indoor environment consistent withembodiments of the invention.

FIG. 18 is a diagrammatic illustration of the MIMO DAS of FIG. 17 inwhich the remote antennas have been located in different areas of theindoor environment and have overlapping coverage.

FIG. 19 is a diagrammatic illustration of the MIMO DAS of FIG. 17 inwhich the remote antennas have been located in different areas of theindoor environment and do not have overlapping coverage.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of embodimentsof the invention. The specific design features of the system and/orsequence of operations as disclosed herein, including, for example,specific dimensions, orientations, locations, and shapes of variousillustrated components, will be determined in part by the particularintended application and use environment. Certain features of theillustrated embodiments may have been enlarged, distorted or otherwiserendered differently relative to others to facilitate visualization andclear understanding.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the drawings, wherein like numbers denote like partsthroughout the several views, FIG. 1 illustrates a schematic view of onepossible implementation of a Multiple-Input/Multiple-Output (“MIMO”)distributed antenna system 10, wherein a MIMO base station (“BTS”) 12 isincorporated in or proximate to an environment in accordance with theinvention. As illustrated in FIG. 1, the system 10 includes a MIMO BTS12 that might be configured with at least two antennas 14 and 16. Whilea 2×2 MIMO BTS is illustrated, one of skill in the art would understandthat additional antennas might be used for a different MIMO scheme. Thefirst antenna 14 is coupled to a first master unit 18 a through a firstsignal link 20 a, while the second antenna 16 is coupled to a secondmaster unit 18 b through a second signal link 20 b. Alternatively, theMIMO BTS 12 might not be configured with antennas. In those embodiments,the MIMO BTS 12 includes at least two antenna ports (not shown) in placeof the antennas 14 and 16. The antennas 15 and 16 may be implementedelsewhere to capture the MIMO signals from another BTS to forward to theMIMO BTS 12. A first antenna port is coupled to the first master unit 18a through the first signal link 20 a, while a second antenna port iscoupled to the second master unit 18 b through the second signal link 20b. The signal links are any appropriate form for passing signals betweenthe components. As illustrated in FIG. 1, the first and second masterunits 18 a-b might be configured as first and second sub-master units 18a-b of a MIMO master unit 19.

The master units 18 a-b are coupled through respective broad bandtransport mediums or links 22 a-b to a plurality of respective remoteunits 24 a-b. Each link 22 a-b might be a high-speed digital link or awideband analogue signal transmission link. For example, an analogtransport medium/link may be used for connecting the remote units 24 a-bwith respective master units 18 a-b. Alternatively, the transport linksmay be implemented as optical links using optical fiber as discussedbelow. With such fiber, the traffic between the remote units 24 a-b andthe master units 18 a-b may be implemented using a radio-over-fiber(“RoF”) format, for example. In this manner, the signals from the masterunits 18 a-b are provided to the remote units 24 a-b in an analogformat, which may assist in preventing at least some degradation due totransmission line effects, which appears in traditional copper-basedtransmission lines. It will be appreciated by one having ordinary skillin the art that filtering may also be used to allow and/or prevent thedistribution of specific signals. As such, and in some embodiments, eachof the links 22 a-b may be a wideband digitally modulated opticalinterface, such as fiber optic cable. Thus, each master unit 18 a-b maybe configured to digitize their respective input signals and outputthose digital signals for their respective remote units 24 a-b. Thesedigital output signals may, in some embodiments, be time divisionmultiplexed into frames and converted into a serial stream. The remoteunits 24 a-b, in turn, may be configured to receive the digital outputsignals from their respective master units 18 a-b, convert the digitaloutput signals into electrical signals, if necessary, de-frame varioustime slots and/or de-serialize the electrical signals, and transmit theelectrical signals via respective local antennas 25 a-b. The masterunits 18 a-b and remote units 22 a-b, in turn, may be controlled by asystem controller 27, which may provide overall supervision and controlof the master units 18 a-b and remote units 22 a-b, as well as alarmforwarding.

FIG. 1 illustrates that the remote units 24 a-b, or at least therespective antennas 25 a-b for the remote units 24 a-b, are disposedwithin an indoor environment 26. As will be appreciated by one havingordinary skill in the art, an indoor environment 26 includes structuresthat can cause obstruction (partial or total) of the radiotransmissions. These include walls, partitions, structural components,electrical conduits, plumbing, doors, computers, and people, forexample. As such, the indoor environment 26 is illustrated fordiscussion purposes as including two areas 28 a-b that are provided withsignals by the respective remote units 24 a-b and that may be at leastsomewhat electromagnetically isolated. For illustration purposes, theareas 28 a-b are illustrated as separated by an illustrative separator30, such as at least one partition or wall 30, though one havingordinary skill in the art will appreciate that the areas 28 a-b may beat least somewhat electromagnetically isolated by way of otherstructures within the indoor environment 26, by the distance between thetwo areas 28 a-b, or in some other manner as will be appreciated by onehaving ordinary skill in the art. Furthermore, environment 26 mightinclude other areas in addition to 28 a-b that affect the signals ofother remote units of the plurality in addition to the remote unites 24a-b that are illustrated for discussion purposes.

Thus, there is an imbalance of the power of signals from the remoteunits 24 a-b received by the wireless devices 32 a-b. For example,remote units 24 a-b can provide signals to wireless device 32 a in itsrespective area 28 a, but there is a power imbalance between the signalsreceived by device 32 a from the respective remote units 24 a-b.Similarly, remote units 24 a-b can provide signals to wireless device 32b in its respective area 28 b, but there is a power imbalance betweenthe signals received by device 32 b from the respective remote units 24a-b. Each wireless device 32 a-b, in turn, may be configured with atleast two antennas 34 a-d to communicate signals to and/or from theremote units 24 a-b according to MIMO schemes. As illustrated in FIG. 1,a first wireless device 32 a is configured with at least two antennas 34a-b, while a second wireless device 32 b is configured with at least twoantennas 34 c-d.

In one embodiment, the remote units 24 a-b are configured to send and/orreceive digital RF voice and/or data signals to and/or from the wirelessdevices 32 a-b via their local antennas 25 a-b. The master units 18 a-bconvert a signal from their respective remote units 24 a-b from anoptical signal to an electrical signal and send the electrical signal tothe antennas 14 and/or 16 of the MIMO BTS 12, which may be configured todetect and receive their respective portions thereof. Alternatively, themaster units 18 a-b may convert a signal from their respective remoteunits 24 a-b from an optical signal to an analog electrical signal,separate the electrical signal into a plurality of electrical signals ina plurality of bands corresponding to those utilized by the MIMO BTS 12,convert the plurality of electrical signals into a plurality of analogsignals, and send the plurality of analog signals to the MIMO BTS 12.

A master unit 18 a-b may be selectively connected to respective remoteunits 24 a-b in a number of ways. For example, master unit 18 a isillustrated as connected to remote unit 24 a through full-duplex link 22a (e.g., a time-division multiplexed link) for uplink and downlink toand from the remote unit 24 a. Master unit 18 b is connected to remoteunit 24 b in a similar manner. However, one having ordinary skill in theart will appreciate that the master units 18 a-b may be connectedthrough two half-duplex links to each respective remote unit 24 a-b. Forexample, and in alternative embodiments, the master unit 18 a can beconnected through a first half-duplex link (not shown) to remote unit 24a for uplink to the remote unit 24 a, and be connected through a secondhalf-duplex link (not shown) to remote unit 24 a for downlink from theremote unit 24 a. Master unit 18 b may be similarly connected to remoteunit 24 b. As illustrated in FIG. 1, in a full-duplex link, the uplinksignals and downlink signals are carried on different wavelengths and awavelength division multiplexer (“WDM”) is employed to combine and/orsplit the two optical signals at the master units 18 a-b and remoteunits 24 a-b. Alternatively, the master units 18 a-b and remote units 24a-b may communicate through a different analog or digital transceiverfor high data rate media such as coax cable, twisted pair copper wires,free space RF or optics, or shared networks such as Ethernet, SONET,SDH, ATM and/or PDH, among others, including one that exploits WDM.

One having skill in the art will appreciate that portions of the system10 might be coupled to a SISO BTS. Accordingly, embodiments of theinvention may be used to retrofit such SISO distributed antenna systems,allowing substantial costs savings using existing SISO equipment toimplement MIMO distributed antenna systems to implement MIMO modes ofoperation in accordance with the aspects of the invention. For example,such a system might include two SISO BTSs that can be replaced with oneMIMO BTS 12 consistent with embodiments of the invention.

As discussed above, the signals provided by the respective remote units24 a-b to the mobile devices 32 a-b in the environment 26 may beassociated with a power imbalance or be at least somewhatelectromagnetically isolated. In accordance with one aspect of theinvention, in the areas where the signals from the respective remoteunits 24 a-b are isolated, the system 10 is configured to utilizemulti-user (“MU”) MIMO techniques to communicate with the wirelessdevices 32 a-b in those isolated areas. However, there may be areaswithin the indoor environment 26 in which the signals from the remoteunits 24 a-b overlap to a certain degree. As such, in one embodiment,the system 10 is configured to utilize single-user (“SU”) MIMOtechniques to communicate with the wireless devices 32 a-b in thoseoverlapping areas. Thus, and in some embodiments, the system 10 isconfigured to dynamically switch between SU-MIMO modes of operation andMU-MIMO modes of operation for sending signals to the wireless devices32 a-b based upon signal quality indicators provided by those wirelessdevices 32 a-b. Thus, 3GPP LTE MIMO features (such as TX diversity, DLSU-MIMO, as well as DL/UL MU-MIMO) may be dynamically used.

It will be appreciated that such an aspect of the invention might besomewhat in contrast to the embodiments of the invention also discussedherein that maintain a certain degree of signal coverage overlappingbetween remote units 12 a-b as requested by downlink SU-MIMO whenimplemented through the system 10. Therefore, for realizing both suchadvantages, embodiments of the invention manage and balance the benefitsof both such MIMO features.

Thus, each remote unit 24 a-b provides signals to, and receives signalsfrom, respective wireless devices 32 a-b present within the respectiveareas 28 a-b. One benefit of this arrangement as noted is that uplinkcollaborative MIMO (for WiMAX) and/or uplink MU-MIMO (for LTE) may beused to increase the total uplink capacity in the system 10 by reusingthe time and/or frequency resources associated with the differentwireless devices 32 a-b. As such, each of the wireless devices 32 a-bmay share resources (e.g., DL/UL MU-MIMO resources) as well as beassociated with a high sector capacity (e.g., again, DL/UL MU-MIMO).

The MIMO BTS 12 is configured with at least one central processing unit(“CPU”) 36 coupled to a memory 38. Each CPU 36 is typically implementedin hardware using circuit logic disposed on one or more physicalintegrated circuit devices or chips. Each CPU 36 may be one or moremicroprocessors, micro-controllers, field programmable gate arrays, orASICs, while memory 38 may include random access memory (RAM), dynamicrandom access memory (DRAM), static random access memory (SRAM), flashmemory, and/or another digital storage medium, and also typicallyimplemented using circuit logic disposed on one or more physicalintegrated circuit devices, or chips. As such, memory 38 may beconsidered to include memory storage physically located elsewhere in theMIMO BTS 12, e.g., any cache memory in the at least one CPU 36. Memory38 includes a scheduler 40 that can be executed by the CPU 36 todynamically switch the operation of the system 10 from a SU-MIMO mode ofoperation to a MU-MIMO mode of operation.

In communications between MIMO BTS 12 and a wireless device 32, thewireless device 32 may provide feedback to the MIMO BTS 12 about thesignals to and/or from that wireless device 32. For example, andconsidering the LTE standard (which is not intended to limit embodimentsof the invention), the uplink feedback provided by the wireless device32 for support of downlink signals from the MIMO BTS 12 can include oneor more performance metrics related to that signal, including a RankIndicator (RI), a Pre-coding Matrix Indicator (PMI), and a ChannelQuality Indicator (CQI). The RI indicates the number of layers (datastreams), which can be supported by the spatial channel experienced atthe wireless device 32. The PMI is then calculated conditioned on theassociated RI, and the CQI is calculated conditioned on the associatedRI and PMI. Typically, a high value CQI is indicative of a channel ofhigh quality. For an RI=1, only one CQI is reported for each reportingunit in frequency because in such a condition only one layer (datastream) can be transmitted by the MIMO BTS 12. On the other hand forRI=2, two CQI are reported for the spatial multiplexing (DL SU-MIMO) asdifferent data streams experience different spatial channels. The PMIindicates the preferred pre-coding candidate for the correspondingfrequency unit and is selected from the possible pre-coding candidatesof Table 1 for the case of two transmitting antennas according to theRI.

TABLE 1 Pre-Coding Codebook for Transmission on Two Antennas CodebookNumber of layers υ index 1 2 0 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}$ $\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}$ 1 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}$ 2 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}$ 3 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}$ —

The CQI might represent a measure of Signal to Interference plus NoiseRatio (SINR), but in fact it is coded in terms of the Modulation andCoding Scheme (MCS) required for a particular error rate probability, ashighlighted in Table 2. As such, the CQI indicates the combination ofthe maximum information data size and the modulation scheme among QPSK,16QAM, and 64QAM, which can provide block error rate not exceeding 0.1(i.e. 10⁻¹) assuming that the reported rank and the reported pre-codingmatrix are applied in the time-frequency resource. With this definitionof CQI, PMI, and RI, the user equipment or mobile device can report themaximum data size that it can receive and demodulate, taking intoaccount its receiver ability.

TABLE 2 CQI Table CQI Code Rate x Index Modulation 1024 Efficiency 0 outof range 1 QPSK  78 0.1523 2 QPSK 120 0.2344 3 QPSK 193 0.3770 4 QPSK308 0.6016 5 QPSK 449 0.8770 6 QPSK 602 1.1758 7 16QAM 378 1.4766 816QAM 490 1.9141 9 16QAM 616 2.4063 10 64QAM 466 2.7305 11 64QAM 5673.3223 12 64QAM 666 3.9023 13 64QAM 772 4.5234 14 64QAM 873 5.1152 1564QAM 948 5.5547

On this feedback basis, the scheduler 40 of the MIMO BTS 12 isconfigured to adapt the downlink transmission mode in order toaccommodate the data reception of a wireless device 32. Specifically,the scheduler 40 might chose either the DL SU-MIMO or the DL MU-MIMOmodes of operation for one or more wireless devices 32. For example, andas discussed above, it is likely that the scheduler's 40 selection amongthese two MIMO schemes mostly relies upon the RI reported by thewireless devices 32, but may instead rely upon the CQI reported by thewireless devices 32 or calculated from the RI and PMI as discussedabove. Indeed the scheduler 40 may decide to boost the data rate for asingle wireless device 32, in case the channel between the MIMO BTS 12and wireless device 32 supports two spatial streams (e.g., RI=2, suchthat the system 10 utilizes a SU-MIMO mode of operation). On the otherhand, the scheduler 40 may allocate the same time-frequency resources totwo different wireless devices 32, which have reported only a singlestream (e.g., RI=1, such that the system 10 utilizes a MU-MIMO mode ofoperation) channel each, in order to improve the overall sectorcapacity. This is because when a wireless device 32 is configured to bein the MU-MIMO transmission mode, only rank-1 transmission can bescheduled to the wireless device 32.

FIGS. 2-4 illustrate components of an exemplary distributed antennasystem for implementing embodiments of the invention. Focusing now on amaster unit 18, FIG. 2 contains a detailed block diagram of the masterunit 18. Each master unit 18 may contain one or more radio channels 110(typically from one to six radio channels 110, each hereinafter referredto as a “path”), one or more digitally modulated optical channels 112(typically from one to four digitally modulated optical channels 112), acontroller 114, a clock generator 116, and an Ethernet switch 118.

In one embodiment, each path, such as 110 a, may be configured to handlea signal to and from the MIMO BTS 12, for example. For a FDD airinterface, the paths 110 a employ a combiner and a duplexer 120 tohandle the uplink signal and the downlink signal. An RF downconverter122 may amplify the received signal from the combiner/duplexer 120 toensure that an A/D converter 124 is fully loaded. The RF downconverter122 sets a center frequency of a band within the A/D converter passband. The wideband A/D 124 digitizes the entire downlink band of the airinterface to ensure all downlink channels are digitized. A resampler 126converts the signal to a complex format, digitally downconverts thefrequency band in some cases, decimates and filters the signal, andresamples it. This reduces the amount of data associated with a downlinksignal, such as 128 a, that has to be transferred over the optical linesand synchronizes the rate of the digitized data to the optical networkbit rate.

The uplink section of the radio channel 110 a sums 120 the uplinksignals, such as signals 129 a-d, for its assigned band from remoteunits 24 coupled to the master unit 18 after they are converted to anelectrical signal. The summation 130 is resampled, interpolated tochange to a different data rate in some cases, and upconverted by theresampler 132 and then converted to an analog form by the D/A converter134. The RF upconverter 136 translates the center frequency of theanalog signal to the appropriate frequency for the air interface andamplifies it. The amplified signal is applied to the combiner/duplexer120 and is routed back to the MIMO BTS 12.

In embodiments utilizing TDD air interfaces, the combiner and duplexerare replaced by a switching function 138 shown in FIG. 2 for example inradio channel 110 b. While the master unit 18 is receiving the downlinksignal, a RF amplifier in the RF upconverter is disabled and a shuntswitch in the switching function 138 may shunt the RF amplifier toground to further reduce leakage. During intervals when the master unit18 is sending the uplink signal to the base station 24, the RF amplifieris enabled, the shunt switch is opened and a series switch in theswitching function 138 may be opened to protect the RF downconverterfrom damage due to high power levels. The switch control timing 144 isdetermined by a master unit controller 114 from the downlink signal 128b. Additionally, a formatter 146 may apply a data compression to reducethe redundant digital information included in the serial data streambefore it is sent to the transmitter in an electro-optical transceiver148. The compression may allow for saving bandwidth or for using a lesscostly transceiver with lower bit rate. The compressed serial data maybe converted into an uncompressed data stream after being received onthe opposite ends in the optical received of 148 by the receiver sideformatter 146.

Each digitally modulated optical channel 112 a-b is composed of aformatter 146 and an electro-optical transceiver 148. On the outgoingside, the formatter 146 blocks, into time division multiplexed frames,the digitized downlink signal 128 a-b along with a customer Ethernet inReduced Media Independent Interface (“RMII”) format 150 a-b, operationand maintenance (“O&M”) data 152 a-c and synchronization information. Inother embodiments, other interfaces such as MII, RMII, GMII, SGMII,XGMII, among others may be used in place of the RMII interface. Theframed data may be randomized by exclusive or'ing (XOR) it with theoutput of a linear feedback shift register to remove long strings oflogic ones or zeros. Other known coding formats such as 8 bit/10 bit or64 bit/66 bit coding may also be used, but may result in a decrease inefficiency in the use of the digital serial link. This digital data isthen converted to a serial stream which is used to modulate an opticaltransmitter within the electro-optical transceiver 148. In a singlefiber implementation, a wavelength division multiplexer (WDM) 149 may beemployed to combine or split the two optical signals.

For incoming signals from the remote units 24, the electro-opticaltransceiver 148 converts the optical signal to an electrical signal. Theformatter 146 phaselocks to the incoming bit stream and generates a bitclock that is phaselocked to the data rate and aligned with the serialdata stream. The formatter 146 then converts the serial stream to aparallel digital data stream, de-randomizes it and performs framesynchronization. It then breaks out the digitized uplink signal for eachband, buffers each band and routes the bands to the appropriate radiochannel 110 a, 110 b, if necessary. Finally, the formatter 146 breaksout the buffers and O&M Ethernet data 152 a-c and the user Ethernet data150 a-b and routes them to the controller 114 and the Ethernet switch118, respectively.

The master unit controller 114 uses locally stored information andinformation from the O&M Ethernet data to configure and control theother blocks in the master unit 18. It also passes this information tothe remote units 24 and reports status of the remote units 24 and themaster unit 18 to the system controller 27. When a radio channel, suchas 110 b, is assigned to a TDD air interface, the master unit controller114 also uses the corresponding downlink signal 128 b to derive TDDswitch control timing 144.

The master unit controller 114 functions to configure individual modulesas well as supervise individual modules. As part of the configurationand supervision functions, the master unit controller 114 is operable todetermine the uplink/downlink switch timing in TDD systems by decodingthe downlink signaling or acquiring it from a different source such asthe time variant uplink Received Signal Strength Indication (“RSSI”), orsome base station clock signal provided from an external source. Thedownlink frame clock in TDMA systems may be determined and distributedby decoding the downlink signaling to allow time slot based functionssuch as uplink or downlink muting, uplink or downlink RSSI measurementswithin time slots, uplink and downlink traffic analysis, etc. The masterunit controller 114 may detect active channels in the RF spectrum toassist in or automatically configure the filter configuration in theresampler 126, 132. Optimal leveling of the individual signals in theresampler may also be determined by measurement of the RSSI of thevarious signals in the downlink RF band. A remote unit controller mayperform similar tasks in the uplink of the remote unit 24.

The clock generator 116 may use a stable temperature compensated voltagecontrolled crystal (“TCVXO”) to generate stable clocks and referencesignals 154 for master unit 18 functional blocks. Although, one ofordinary skill in the art will appreciate that other devices or crystalsmay also be used to generate clocking signals as long as they arecapable of producing the stable clocks required by the system.

Focusing now on a remote unit 24, FIG. 3A and FIG. 3B contain a detailedblock diagram of a remote unit 24 consistent with embodiments of theinvention. Each unit 24 may contain one or more radio channels 160(typically from one to six radio channels 160), one or more DMOCs 162(typically one or two DMOCs 162), a remote unit controller 164 and anEthernet switch 166.

The DMOCs 162 may be designated as the downstream 168 and upstreamchannels 170. The downstream channel 168 is connected to a remote unit24 that precedes this remote unit 24 in a daisy chain, if so configured.The upstream channel 170 is connected to a master unit 18 or anotherremote unit 24. The DMOC 162 functional blocks are similar to those inthe master unit 18. Both consist of a formatter 172 and electro-opticaltransceiver 174. Outgoing data is buffered, formatted into frames,randomized, parallel to serial converted and used to modulate an opticaltransmitter in the electro-optical transceiver 174. Incoming data isconverted from an optical to electrical format, bit synchronized,de-randomized, frame synchronized and converted to a parallel format.The various data types are then broken out buffered and distributed toother function blocks within the remote unit 24. In some embodiments,formatter 172 may implement compression and decompression schemes toreduce bandwidth over the digital optical link.

Radio channels in the remote unit 24 are functionally similar to thosein the master unit 18. Each radio channel is configured to handle asingle RF band. Unlike the master unit 18 radio channels 110, the remoteunit 24 radio channels 160 are connected via a cross band coupler 176 toits antenna 25. For FDD air interfaces, the radio channels, such asradio channel 160 a, employ a duplexer 178 to split the uplink and thedownlink signal. Duplexers, cross-band combiners and couplers may beoptional for some embodiments of either the master unit 18 or remoteunits 24. In these embodiments, additional antennas may replace theduplexer 178 and cross-coupler 176 in the remote units 42. Extra cableswould be required in the master unit 18. An RF downconverter 180amplifies the received uplink signal from the antenna 25 to ensure anA/D converter 182 is fully loaded and sets the center frequency of theband within the A/D converter pass band. The wideband A/D 182 digitizesthe entire uplink band of the air interface to ensure all uplinkchannels are digitized. A resampler 184 converts the uplink signal to acomplex format, digitally downconverts the signal in some cases,decimates and filters the signal, and resamples it with a multi-ratefilter bank. This reduces the amount of data that has to be transferredover the optical links and synchronizes the rate of the digitized datato the optical network bit rate. The output of the resampler 184 isadded to the uplink signals 186 a from the downstream remote units 24 insummer 187. The summed uplink signal 188 a for each band is then sent toa formatter 172 in the upstream channel 170 in the DMOC 162.

The downlink signal 190 for each band (190 a, 190 b) is interpolated andfrequency shifted in the resampler 192. The group delay of individualspectral components can be adjusted via filters or delay elements in theresampler 192. The signal is then converted to an analog form by the D/Aconverter 194. The RF upconverter 196 translates the center frequency ofthe analog downlink band to the appropriate frequency for the airinterface and amplifies it. The amplified signal is then applied to theantenna 25 and transmitted to a wireless device 32.

For TDD air interfaces, the duplexer 178 is replaced by the switchingfunction 138 shown in radio channel 160 b and FIG. 3A. While the remoteunit 24 is receiving the uplink, the RF power amplifier in the RFupconverter 196 is disabled and a shunt switch in the switching function138 shunts the RF power amplifier to ground to further reduce leakage.When the remote unit 24 is transmitting the downlink signal, the RFpower amplifier is enabled, the shunt switch is opened to permit thedownlink signal to reach the antenna 25 and a series switch in theswitching function 138 is opened to protect the RF downconverter 180from damage due to high power levels. As with the master unit 18, theswitch control timing 144 is determined by the controller 164 from thedownlink signal 190 a, 190 b.

The clock generator 198 includes a voltage-controlled crystal oscillator(“VCXO”) that is phaselocked to the incoming serial data stream bit ratevia a narrowband phaselocked loop (“PLL”). The VCXO output is split andis used as the frequency reference 200 for the local oscillators in eachradio channel 160 a-b, the sampling clocks for the A/D 182 and D/A 194converters, and a clock for the other blocks in the remote unit 24. Oneof ordinary skill in the art will realize that the long term frequencyaccuracy should be good to ensure the local oscillators are on frequencyand that the short term jitter levels should also be low to ensure thatthe jitter does not corrupt the A/D and D/A conversion processes. Byphaselocking to the data rate of the optical link, which is derived fromthe stable TCVCXO in the master unit 18, the remote unit 24 does notrequire an expensive oven compensated oscillator or a GPS discipliningscheme to maintain long term frequency accuracy, thereby, making themore numerous remote units 24 less expensive. The use of a narrow bandPLL and a crystal controlled oscillator may assist in reducing shortterm jitter for the A/D and D/A converter clocks. Using the recovered,jitter reduced clocks 202 to re-clock the transmit data in the opticallinks at each remote unit 24 reduces jitter accumulation which mayassist in improving A/D and D/A converter clocks in the downstreamremote units 24 and may assist in reducing the bit error rate (“BER”) ofthe optical communication channels 162.

The remote unit controller (RUC) 164 uses locally stored information andinformation from the O&M Ethernet to configure and control the otherblocks in the remote unit 24. Downstream RMII 152 d and upstream RMII152 e may also be supplied to the formatter 172. In addition, local O&Mdata 206 may be configured at a local O&M terminal 204. Remote unit 24also passes this information to the up and downstream remote units 24and/or master unit 18. The RUC 164 additionally uses the appropriatedownlink signal to derive TDD switch control timing 144 when required.

In an alternate embodiment of the radio channel 160 c utilized in aremote unit 24, the radio channel 160 c may also employ digitalpre-distortion to linearize the power amplifier. This embodiment of theradio channel 160 c in a remote unit 24 is shown in the block diagram ofFIG. 4. In this embodiment, a third signal path may be added to one ormore radio channels 160 c. The third path couples off the downlinksignal after power amplification and digitizes it. The signal from theantenna 25 is received in an RF downconverter 208, which amplifies thereceived signal to ensure an A/D converter 210 is fully loaded and setsthe center frequency of the band within the A/D converter pass band. Thewideband A/D 210 digitizes the entire uplink band of the air interfaceto ensure all uplink channels are digitized. The digitized signal iscompared to a delayed version of the downlink signal in the digitalpre-distortion unit 212 and the difference is used to adaptively adjustthe gain and the phase of the signal prior to D/A conversion to correctfor non-linearity in the power amplifier.

In some embodiments, the topology of a system 10 can be adjusted tooptimize MIMO channel capacity. For example, Eq. 1 illustrates a MIMOchannel capacity formula for an N×M MIMO system with equal powerallocation to each antenna 25:

MIMO  Channel  Capacity  Formula $\begin{matrix}{C = {{\log_{2}{\det ( {\underset{= N}{I} + {\frac{\rho}{M}\overset{\_}{\underset{\_}{\underset{\_}{H}}}{\overset{\_}{\underset{\_}{\underset{\_}{H}}}}^{H}}} )}} = {\sum\limits_{k = 1}^{R}{\log_{2}( {1 + {\frac{\rho}{M}\lambda_{k}}} )}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Thus, the MIMO channel capacity depends on several parameters that haveto be taken into account for its optimization. First, the number of Nreceiving and M transmitting antennas involved. Second, p represents thesignal-to-noise and interference ration (SNIR) averaged over thereceiving antennas. Finally the H MIMO channel matrix includes thedifferent H_(ij) channel transfer functions between the “i” receivingand “j” transmitting antennas. Furthermore, the MIMO channel matrix isnormalized so that the path-loss effect on its coefficients is removedand included into the SNIR parameter. As a result, the MIMO channelmatrix is only affected by the level of correlation experienced at theantennas. Moreover the MIMO capacity formula can also be written interms of the Eigen-values λ_(k) of the MIMO channel matrix, with kranging from 1 to the MIMO channel matrix rank R.

In some embodiments, the Eigen-values represent an indicator of thecorrelation affecting the MIMO channel. As such, they provide a measureof the MIMO channel's ability to support multiple spatial streams inorder to increase the resulting capacity. Moreover, the channelcondition number (CCN), which is the ratio between the smallestEigen-value and largest Eigen-value, can be exploited as an additionalparameter to measure how conditioned the MIMO channel matrix is. Inother words, for well-conditioned channel matrices, the CCN approachesthe 0 dB value, which means the Eigen-values are all equal and spatialmultiplexing can be successfully exploited by virtue of low correlation(e.g., the system can utilize SU-MIMO modes of operation). On the otherhand, for ill-conditioned matrices the CCN can jump to 20 dB or evenmore, which means the channel is highly correlated and it is not able tosupport spatial multiplexing (e.g., the system cannot utilize SU-MIMOmodes of operation).

For example, FIG. 5 is a graph 300 illustrating the channel capacitypercentage increase as a function of the SNIR, and that furtherillustrates, for a given environment, that the optimization of MIMOchannel capacity depends upon the optimization of the SNIR and the CCN.Specifically, the graph 300 was generated with a 2×2 MIMO systemdeployed in an indoor environment, such as illustrated in FIG. 1. TheCCN provides an indication on the correlation level affecting the MIMOchannel and depends upon several factors, including the scatteringproperties of the specific environment involved (e.g., rich or poor),the MIMO Tx antenna array spacing (e.g., ranging from λ/2 up), theantenna polarization, the Tx and Rx position (e.g., Line of Sight [LoS]positioning, Not Line of Sight [NLoS] positioning), as well as otherfactors. For example, three general indoor environments include denseenvironments (e.g., filled with objects), open environment (e.g.,generally devoid of objects), and large environments (e.g., associatedwith large distances between antennas). FIG. 6 is an illustration of agraph 310 that shows the typical CCN distribution in a “dense” indoorenvironment with co-polarized antennas, FIG. 7 is an illustration of agraph 320 that shows the typical CCN distribution in an “open” indoorenvironment with co-polarized antennas, and FIG. 8 is an illustration ofa graph 330 that shows the typical CCN distribution in a “large” indoorenvironment with co-polarized antennas.

Thus, embodiments of the invention are utilized to keep the channelcorrelation and resulting CCN low by selectively placing antennas (e.g.,remote units) in an environment based on the imbalance of received powerand SNIR associated with those antennas. Specifically, the antennas areplaced in an environment such that wireless devices can receive powercontributions from at least two antennas throughout the environment.More specifically, embodiments of the invention specify the powerimbalance as well as the SNIR required to have a particular capacitywithin that area. Thus, wireless devices receive substantial powercontributions from several antennas deployed throughout the environment.In ideal embodiments, the antenna deployment provides wireless deviceswith LoS channel conditions from each antenna throughout the environmentsuch that both low spatial correlation and high SNIR conditions areachieved. However, this solution is often associated with high costs dueto the large number of antennas that may be necessary.

FIG. 9 is an illustration of an indoor environment 400 in which fourantennas 25 a, 25 a′, 25 b and 25 b′ have been distributed, and in whichvarious imbalances between two signals of a 2×2 MIMO DAS (e.g., such asthe system 10 of FIG. 1) have been measured. For example, the powerimbalance of received signals is determined by a wireless device or amobile test set-up (or equivalent equipment). Specifically, the deviceor test set-up determines the received power of a first signal, thereceived power of a second signal, and determines the absolute value ofthe difference of the received power of those two signals to determinethe power imbalance of the received signals. With respect to FIG. 1,like reference numerals are utilized in FIG. 9 where applicable.

Antennas 25 a and 25 a′ are configured to communicate a first MIMOsignal, while antennas 25 b and 25 b′ are configured to communicate asecond MIMO signal. In some embodiments, each antenna 25 a, 25 a′, 25 band 25 b′ are connected to a respective remote units 24 (not shown),while in alternative embodiments antennas 25 a and 25 a′ are connectedto a first remote unit 24 a and antennas 25 b and 25 b′ are connected toa second remote unit 24 b. A wireless device or mobile test set-up (orequivalent equipment) (not shown) may then determine the power imbalancebetween the first and second received signals as shown at theillustrated data points. As illustrated in FIG. 9 by referencemeasurements 402, the interleaved antennas 25 a, 25 a′, 25 b and 25 b′provide good coverage uniformity and substantial overlapping betweencoverage areas. For a measured point 402, the relation between thereceived power imbalances from the signals from the antennas 25 a or 25a′ and the antennas 25 b or 25 b′ correspond to a particular CCN.

For example, FIG. 10 is a graph 410 illustrating the effect of animbalance of the received power of two signals on the CCN when thatimbalance is less than about 5 dB, while FIG. 11 is a graph 420illustrating the effect of an imbalance of the received power of twosignals on the CCN when that imbalance is from about 5 dB to about 10dB. Similarly, FIG. 12 is a graph 430 illustrating the effect of animbalance of the received power of two signals on the CCN when thatimbalance is from about 10 dB to about 15 dB, while FIG. 13 is a graph440 illustrating the effect of an imbalance of the received power of twosignals on the CCN when that imbalance is greater than about 15 dB.Thus, for a low power imbalance (e.g., less than 5 dB) the CCN assumes alow value, while a high power imbalance (e.g., more than 15 dB) isassociated with a CCN of 20 dB or more. By adopting a MIMO antennadistribution approach of the invention, the CCN is driven by the powerimbalance parameter rather than channel correlation.

To determine where to selectively place antennas in accordance with oneaspect of the invention, embodiments of the invention determines theimbalance of the received power from two interleaved antennas, as wellas the SNIR experienced at that point. A user determines the CCN fromthat power imbalance, then correlates the CCN and SNIR to a datastore(e.g., a database, graph, or other collection) of information todetermine the MIMO capacity of a MIMO system with the antennas at theirselected locations. For example, FIG. 14 is datastore in the form of agraph 450 illustrating the effect of the CCN and SNIR with the capacityof a MIMO system with interleaved antennas according to the invention.By determining the CCN and SNIR at a point, a user determines thecapacity for a MIMO system at a particular point in the environment, andthus determines whether to place an antenna at that point or to place itat an alternative point in the area.

FIG. 15 is a flowchart 500 illustrating a sequence of operations toselectively determine a location to deploy a plurality of remote units,or a plurality of antennas of remote units, to optimize the capacity ofa MIMO DAS in an environment consistent with embodiments of theinvention. Initially, a user deploys a first antenna at a first locationthat may be desirable from a signal coverage standpoint (block 502).Then a user deploys a second antenna at a second location (block 504).An imbalance between the received power of signals from the firstantenna and the received power of signals from the second antenna isthen determined at a predetermined location (e.g., a “power imbalance”)(block 506).

After determining the power imbalance, a CCN and SNIR for the signals atthe predetermined location is determined (block 508) and the capacity ofthe MIMO DAS with the first antenna at the first location and the secondantenna at the second location is determined (block 510). Specifically,and as described above, a datastore of the relationship between the CCNand the SNIR may be utilized to determine the capacity of the MIMO DAS.As such, when the capacity is not acceptable (e.g., the capacity of theMIMO DAS is not high enough for a desired installation) (“No” branch ofdecision block 512), the user adjusts the location of deployment of thesecond antenna (block 514) and the sequence of operations returns toblock 506. However, when the capacity is acceptable (e.g., the capacityof the MIMO DAS is high enough for a desired installation) (“Yes” branchof decision block 512) the sequence of operations ends.

In alternative embodiments, a determined power imbalance can be utilizedto tune a MIMO DAS to more efficiently operate in a SU-MIMO mode ofoperation or a MU-MIMO mode of operation. FIG. 16 is a flowchart 520illustrating a sequence of operations for a user to selectively tune theoperation of a MIMO DAS in either a SU-MIMO mode of operation or aMU-MIMO mode of operation based upon a power imbalance consistent withembodiments of the invention. Initially, the user deploys a firstantenna at a first location (block 522) and deploys a second antenna ata second location (block 524). A power imbalance between signals fromthe first and second antennas is then determined at a predeterminedlocation (block 526).

The power imbalance between signals from the first and second antennascan be used to tune a MIMO DAS to more efficiently utilize SU-MIMO andMU-MIMO modes of operation. As such, it is determined whether the powerimbalance is below a predetermined threshold, such as about 15 dB (block528). When the power imbalance is below the predetermined threshold(“Yes” branch of decision block 528) it is determined whether the MIMODAS is configured to utilize SU-MIMO modes of operation (block 530).When the MIMO DAS is not going to be utilized with SU-MIMO modes ofoperation (“No” branch of decision block 530), the location of thedeployment of the second antenna is adjusted to increase the powerimbalance (block 532) and the sequence of operations returns to block526.

When the MIMO DAS is configured to utilize SU-MIMO modes of operation(“Yes” branch of decision block 530), it is determined whether thecapacity for the MIMO DAS is acceptable (block 534). When the capacityis not acceptable (“No” branch of decision block 534) the location ofthe deployment of the second antenna is adjusted to increase thecapacity of the MIMO DAS (block 536). However, when the capacity isacceptable (“Yes” branch of decision block 534) the sequence ofoperations ends.

Returning to block 528, when the imbalance is not less than thepredetermined limit (“No” branch of decision block 528), it isdetermined whether the MIMO DAS is configured to utilize MU-MIMO modesof operation (block 538). When the MIMO DAS is not configured to utilizeMU-MIMO modes of operation (“No” branch of decision block 538) thelocation of the deployment of the second antenna is adjusted to decreasethe power imbalance (block 540). However, when the MIMO DAS isconfigured to utilize MU-MIMO modes of operation (“Yes” branch ofdecision block 538), it is again determined whether the capacity for theMIMO DAS is acceptable (block 534). When the capacity is not acceptable(“No” branch of decision block 534) the location of the deployment ofthe second antenna is adjusted to increase the capacity of the MIMO DAS(block 536). However, when the capacity is acceptable (“Yes” branch ofdecision block 534) the sequence of operations ends.

As discussed above, the CCN corresponds to the RF power imbalance of twosignals. Specifically, in a 2×2 MIMO system, the interleaving ofantennas using a DAS signal gives an advantage in terms of the capacity,C, when compared to classical MIMO deployments based on coverage with aplurality of antennas as well as co-located antenna arrays. Thus, theposition of each remote unit or antenna is the driver for building radiocoverage within an environment to exploit the maximum capacity C inaccordance with the invention.

In some embodiments, users can employ a ray-tracing simulator,algorithm, or other equivalent simulation to determine the optimizedposition of each antenna to provide a maximum C within an environment orto optimize the operation of a system for either MU-MIMO or SU-MIMOmodes of operation. As such, embodiments of the invention, andparticular embodiments of the invention that utilize the sequence ofoperations illustrated in FIG. 15 and FIG. 16, can be performed by acomputing system, or otherwise integrated into program code executed bythe computing system. Such a computing system typically includes one ormore processors coupled to a memory. The computing system also typicallyincludes at least one network interface coupled to at least one network,as well as at least one input/output device interface coupled to atleast one peripheral device, such as a user interface (including, forexample, a keyboard, mouse, a microphone, and/or other user interface)and/or at least one output device (including, for example, a display,speakers, a printer, and/or another output device). Such computersystems are often under the control of an operating system and execute,or otherwise rely upon, various computer software applications,sequences of operations, components, programs, files, objects, modules,etc., consistent with embodiments of the invention.

Alternatively, users can take advantage of the RF power imbalance,specifically of a pre-installed SISO system. For example, a first remoteunit or antenna may be placed inside an environment according to wellknow radio coverage design rules (e.g., for example, in an alreadyinstalled SISO system).

To exploit capacity and capabilities of a MIMO system, a second remoteunit may be placed in a different position to achieve, from the twodifferent paths and for the whole in-building area or other environmentunder consideration, an RF power imbalance below to a given limit, suchas about 15 dB, for example. More specifically, the proper placement ofthe second remote unit can be determined at least three different ways:(1) exploiting SISO radio coverage design rules with the goal tomaximize the area of the environment where the RF power imbalance isbelow a predetermined limit (for example, using a SISO radio coverage SWtool); (2) running different trials in which different locations for thesecond antenna are attempted and exploiting a wireless device or othermobile test-set (or equivalent equipment) to maximize the coverage areawhere the RF power imbalance is below the predetermined limit, includingfinding the location for the second antenna that takes advantage of thescattering or shadowing effect of the environment; or (3) if it isinfeasible to try several locations for the second antenna, anapproximate location for the second antenna can be used and, from thesame wireless device or other mobile test-set (or equivalent equipment),information on the RF power imbalance and SNIR from the first remoteunit can be gathered in order to delimit/analyze the coverage area wherea particular capacity C for the MIMO system can be guaranteed.

As such, a user may determine a desired layout of antennas and/or remoteunits throughout the target environment based upon the existing coverageof that environment, the coverage that can be provided by antennasand/or remote units, and cost considerations. In some embodiments, thisdetermination can be made by analyzing known and/or potential coverages,capacities, and costs of purchasing, installing, and maintainingequipment (remote units, cabling therefore, etc.). The user then selectsa layout that provides the desired coverage with the desired capacitywithin a desired budget.

By way of example, FIG. 17 is a diagrammatic illustration of a MIMO DAS600 that includes co-located remote antennas 25 a-b within anenvironment 602. With respect to FIG. 1, like reference numerals areutilized in FIG. 17-19 where applicable. Specifically, the MIMO DAS 600includes a 2×2 MIMO BTS 12 that provides respective MIMO signals torespective master units 18 a-b. The antennas 25 a-b can be connected torespective remote units 24 a-b (not shown) or connected directly to themaster units 18 a-b. As illustrated, the MIMO DAS 600 provides aparticular coverage area 604 with the antennas 25 a-b. For example, thecoverage area 604 in the environment 602 may provide signals to only oneof three wireless devices 606 a-c, with wireless devices 606 a and 606 cunable to receive signals from either antenna 602 a-b (as they are bothoutside of coverage area 604) and wireless device 606 b receivingsignals from both antennas 25 a-b. As such, the wireless device 606 bcan utilize SU-MIMO modes of operation, and thus experience a data rateboost, as well as experience transmit diversity against fast fading.However, this particular setup provides the smallest coverage for theenvironment 602, provides no coverage for wireless devices 606 a and 606c, results in a high correlation, and also has a limited sector capacityfor either DL or UL MU-MIMO.

FIG. 18, on the other hand, is a diagrammatic illustration of a MIMO DAS610 in which the remote antennas 25 a-b are distributed within theenvironment 602 but have overlapping coverage areas 612 a-b.Specifically, FIG. 18 illustrates that wireless devices 606 a and 606 bare within the coverage area 612 a for the first antenna 25 a, whilewireless devices 606 b and 606 c are within the coverage area 612 b forthe second antenna 25 b. Thus, there is low correlation and transmitdiversity against slow fading for wireless device 606 b, which canutilize SU-MIMO modes of operation. However, there is also SISO coveragefor wireless devices 606 a and 606 c, which can share resources andutilize MU-MIMO modes of operation. Thus, FIG. 18 illustrates that ahigh sector capacity for MU-MIMO is achieved by distributing theantennas 25 a-b. However, this configuration results in a limited datarate boost for wireless device 606 b and does not provide transmitdiversity against fast fading.

FIG. 19 is a diagrammatic illustration of a MIMO DAS 620 in which theremote antennas 25 a-b are distributed within the environment 602 but donot have overlapping coverage areas 622 a-b. FIG. 19 illustrates thatwireless devices 606 a is within coverage area 622 a, that wirelessdevice 606 b is within either coverage area 622 a or 622 b (but not bothat the same time), and that wireless device 606 c is within coveragearea 622 b. However, the coverage areas 622 a and 622 b do not overlap.Thus, the largest SISO coverage for the environment 602 is provided.Specifically, there is SISO coverage for each wireless device 606 a-c,while wireless devices 606 a and 606 c can share resources and utilizeMU-MIMO modes of operation. The MIMO DAS 620 also has a higher sectorcapacity for MU-MIMO modes of operation than either the MIMO DAS 600 ofFIG. 17 or the MIMO DAS 610 of FIG. 18. However, wireless device 606 bdoes not have a data rate boost and cannot operate in SU-MIMO modes ofoperation, as it only receives signals from one antenna 252 a or 252 b.Moreover, there is no transmit diversity for any of the wireless devices606 a-c.

While the present invention has been illustrated by the description ofthe embodiments thereof, and while the embodiments have been describedin considerable detail, it is not the intention of the applicant torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. For example, a distributed antenna systemconsistent with embodiments of the invention may have more or fewer MIMOBTSs 12, master units 18, remote units 24, and/or system controllers 27than those illustrated. In particular, each MIMO BTS 12 may include moreor fewer antennas 14 and/or 16.

Additionally, each master unit 18 may be connected to more or fewerremote units 24 than those illustrated. As such, a plurality of remoteunits 24 may be connected to each master unit 18 through two linksand/or along a single link as discussed above. Alternatively, eachremote unit 24 may be connected to a master unit 18 through a dedicatedlink. In some embodiments, a plurality of remote units 24 may beconnected in series from a master unit 18. As such, remote units 24 maybe positioned to optimize coverage within a coverage area consistentwith embodiments of the invention. Moreover, one having ordinary skillin the art will appreciate that a master unit 18 may be incorporatedwith a remote unit 24, and thus operate as an active (or passive)distribution point as is well known in the art. As such, each suchmaster unit 18 may be connected directly to at least one antenna 25.Moreover, one having ordinary skill in the art will appreciate that inthe links 22 a-b connecting the master units 18 to the remote units 24,a passive (or active) power splitter can be inserted in order to deployadditional remote units. As such, each such master unit 18 input/outputport may be coupled with a plurality of remote units 24 consistent withembodiments of the invention.

Furthermore, and in some embodiments, the master unit controller 114 maymeasure a pilot signal strength of CDMA or Orthogonal Frequency-DivisionMultiplexing (“OFDM”) signals to properly set the level of the downlinksignals, as the RSSI can vary at different capacity loading. The pilotsignals generally remain constant with a configured ratio between pilotlevel and a maximum composite for full loading, the required headroomfor the signals may be maintained. The master unit controller 114 mayalso measure and supervise the signal quality of the provided downlinkchannels. In case of signal degradation, an alarm may be set and theoperator can focus on a base station (e.g., the MIMO BTS 12) withouthaving to troubleshoot the entire system 10.

In some embodiments, the master unit controller 114 determines theamount of channels for a narrowband base station standard such as GlobalSystem for Mobile communications (“GSM”). Together with the measurementof the Broadcast Control Channel (“BCCH”), which is constant in power,the proper headroom that is required for a multichannel subband may bedetermined and overdrive or underdrive conditions may be avoided. Inother embodiments, the master unit controller 114 monitors the crestfactor of a transmitted spectrum in the presence of multiple channels.The crest factor may provide input to the leveling of the transmit poweror the power back-off of particular gain stages of the system. Theconfigured headroom is generally higher than the measured crest factorto avoid signal degradation due to clipping or distortion. In addition,a crest factor reduction mechanism may be employed in the resampler insome of the embodiments to reduce the crest factor and make moreefficient use of the RF power amplifier in the remote unit 24 or assistin reducing the number of required bits per sample that need to betransmitted over the link.

Some embodiments of the invention provide benefits in regard to theuplink path of a MIMO communication system. Both WiMAX and LTE wirelessstandards encompass uplink MIMO features. In particular the “UplinkCollaborative MIMO” is implemented in Mobile WiMAX, while “UplinkMU-MIMO” is the term adopted in LTE for indicating the same technique.The peculiarity of this MIMO scheme is to increase the total uplinksector capacity by reusing time/frequency resources allocated todifferent wireless devices 32, rather than to boost the data rate persingle user as for downlink SU-MIMO (Spatial Multiplexing).

The routines executed to implement embodiments of the invention, whetherimplemented as part of an operating system or a specific application,component, scheduler, program, object, module or sequence ofinstructions executed by one or more computing systems have beenreferred to herein as a “sequence of operations,” a “program product,”or, more simply, “program code.” The program code typically comprisesone or more instructions that are resident at various times in variousmemory and storage devices, and that, when read and executed by one ormore processors, cause that a system associated with that processor toperform the steps necessary to execute steps, elements, and/or blocksembodying the various aspects of the invention.

While the invention has been described in the context of fullyfunctioning devices, those skilled in the art will appreciate that thevarious embodiments of the invention are capable of being distributed asa program product in a variety of forms, and that the invention appliesequally regardless of the particular type of computer readable signalbearing media used to actually carry out the distribution. Examples ofcomputer readable signal bearing media include but are not limited tophysical and tangible recordable type media such as volatile andnonvolatile memory devices, floppy and other removable disks, hard diskdrives, optical disks (e.g., CD-ROM's, DVD's, etc.), among others, andtransmission type media such as digital and analog communication links.

In addition, various program code that has been described may have beenidentified based upon the application or software component within whichit is implemented in a specific embodiment of the invention. However, itshould be appreciated that any particular program nomenclature is usedmerely for convenience, and thus the invention should not be limited touse solely in any specific application identified and/or implied by suchnomenclature. Furthermore, given the typically endless number of mannersin which computer programs may be organized into routines, procedures,methods, modules, objects, and the like, as well as the various mannersin which program functionality may be allocated among various softwarelayers (e.g., operating systems, libraries, APIs, applications, applets,etc.), it should be appreciated that the invention is not limited to thespecific organization and allocation of program functionality describedherein.

Thus, the invention in its broader aspects is not limited to thespecific details representative apparatus and method, and illustrativeexamples shown and described. Accordingly, departures may be made fromsuch details without departure from the spirit or scope of theapplicants' general inventive concept. For example, the system 10 ofFIG. 1 may be configured with an extension unit (not shown) disposedbetween a master unit 18 and its corresponding remote units 24. Theextension unit may provide additional links for coupling a master unit18 to additional remote units 24 and/or the extension unit may extendthe range of coupling between a master unit 18 and remote units 24.

Additionally, it will be appreciated that the environments 26, 400, and602 are merely included to show operation of embodiments of theinvention therewith, and that embodiments of the invention may be usedwith indoor or outdoor environments without departing from the scope ofthe applicants' general inventive concept. Furthermore, in someembodiments, the indoor environment 26 of FIG. 1 and the indoorenvironment 400 of FIG. 9 are configured in alternative manners thanthose illustrated.

Other modifications will be apparent to one of ordinary skill in theart. Therefore, the invention lies in the claims hereinafter appended.

What is claimed is:
 1. A method for determining placement of a pluralityof simulated antennas of a simulated distributed antenna system forhandling simulated MIMO signals in a simulated environment comprising:at a first simulated location within the simulated environment,simulating communication of a first simulated MIMO signal by a firstremote unit over a first simulated air interface in the simulatedenvironment; at a second simulated location within the simulatedenvironment, simulating communication of a second simulated MIMO signalby a second remote unit over a second simulated air interface in thesimulated environment; the first simulated location and the secondsimulated location arranged within the simulated environment to provideoverlapping simulated signal coverage of both the first simulated MIMOsignal and the second simulated MIMO signal at a third simulatedlocation in the simulated environment; and analyzing at least asimulated imbalance of simulated received power between the firstsimulated MIMO signal and the second simulated MIMO signal within thesimulated environment at a third simulated location in order todetermine whether a desired capacity for simulated MIMO communicationswith the simulated distributed antenna system is achieved at the thirdsimulated location in the simulated environment.
 2. The method of claim1, wherein at least one of simulating communication of the firstsimulated MIMO signal and simulating communication of the secondsimulated MIMO signal in the simulated environment includes positioninga simulated remote unit at a respective simulated location within thesimulated environment and simulating communication of a respectivesimulated MIMO signal over a respective simulated air interface.
 3. Themethod of claim 1, further comprising: determining a simulatedsignal-to-noise and interference ratio associated with at least one ofthe first simulated MIMO signal and the second simulated MIMO signal atthe third simulated location in the simulated environment.
 4. The methodof claim 3, further comprising: determining a simulated channelcondition number associated with the simulated distributed antennasystem from the analyzed simulated imbalance.
 5. The method of claim 4,further comprising: determining a simulated capacity for the simulateddistributed antenna system from the simulated signal-to-noise andinterference ratio and the simulated channel condition number.
 6. Themethod of claim 5, further comprising: determining that the simulatedcapacity for the simulated distributed antenna system does not meet apredetermined capacity threshold; and adjusting the simulated locationof deployment of the simulation of at least one of the simulated MIMOsignals.
 7. The method of claim 1, further comprising: determiningwhether the analyzed simulated imbalance is less than a predeterminedthreshold.
 8. The method of claim 7, further comprising: in response todetermining that the analyzed simulated imbalance is less than thepredetermined threshold, determining whether to increase or decrease theanalyzed simulated imbalance.
 9. A computing system configured tosimulate determination of placement of a plurality of simulated antennasof a simulated distributed antenna system for handling simulated MIMOsignals in a simulated environment, the computing system comprising: atleast one processor; and at least one memory communicatively coupled tothe at least one processor; the at least one processor configured tosimulate communication of a first simulated MIMO signal from a firstremote unit at a first simulated location over a first simulated airinterface in the simulated environment; the at least one processorfurther configured to simulate communication of a second simulated MIMOsignal from a second remote unit at a second simulated location over asecond simulated air interface in the simulated environment; the firstsimulated location and the second simulated location arranged within thesimulated environment to provide overlapping simulated signal coverageof both the first simulated MIMO signal and the second simulated MIMOsignal at a third simulated location in the simulated environment; andthe at least one processor further configured to analyze at least animbalance of simulated received power between the first simulated MIMOsignal and the second simulated MIMO signal within the simulatedenvironment at a third simulated location in order to determine whethera desired capacity for simulated MIMO communications with the simulateddistributed antenna system is achieved at the third simulated locationin the simulated environment.
 10. The computing system of claim 9,wherein the at least one processor being configured to at least one ofsimulate communication of the first simulated MIMO signal and simulatecommunication of the second simulated MIMO signal in the simulatedenvironment includes the at least one processor being configured toposition a simulated remote unit at a respective simulated locationwithin the simulated environment and to simulate communication of arespective simulated MIMO signal over a respective simulated airinterface.
 11. The computing system of claim 9, wherein the at least oneprocessor is further configured to determine a simulated signal-to-noiseand interference ratio that is associated with at least one of the firstsimulated MIMO signal and the second simulated MIMO signal.
 12. Thecomputing system of claim 11, wherein the at least one processor isfurther configured to determine a simulated channel condition numberassociated with the simulated distributed antenna system from theanalyzed simulated imbalance.
 13. The computing system of claim 12,wherein the at least one processor is further configured to determine asimulated capacity of the simulated distributed antenna system from thesignal-to-noise and interference ratio and the channel condition number.14. The computing system of claim 13 wherein the at least one processoris further configured to compare the simulated capacity for thesimulated distributed antenna system to a predetermined capacitythreshold.
 15. The computing system of claim 13, wherein the at leastone processor is further configured to access a datastore includingcapacity information.
 16. The computing system of claim 13, wherein theat least one processor is further configured to determine whether toincrease or decrease the simulated imbalance in response to determiningthat the simulated imbalance is less than the predetermined threshold.17. The computing system of claim 13, wherein the at least one processoris further configured to: determine that the simulated capacity for thesimulated distributed antenna system does not meet a predeterminedcapacity threshold; and adjust the simulated location of deployment ofthe simulation of at least one of the simulated MIMO signals.
 18. Thecomputing system of claim 13, wherein the at least one processor isfurther configured to: determine whether the analyzed simulatedimbalance is less than a predetermined threshold; and determine whetherto increase or decrease the analyzed simulated imbalance in response todetermining that the analyzed simulated imbalance is less than thepredetermined threshold.
 19. An apparatus for simulating determinationof placement of a plurality of simulated antennas of a simulateddistributed antenna system for handling simulated MIMO signals in asimulated environment, the apparatus including: at least one circuitconfigured to simulate communication of a first simulated MIMO signalfrom a first remote unit at a first simulated location over a firstsimulated air interface in the simulated environment; the at least onecircuit further configured to simulate communication of a secondsimulated MIMO signal from a second remote unit at a second simulatedlocation over a second simulated air interface in the simulatedenvironment; the first simulated location and the second simulatedlocation arranged within the simulated environment to provideoverlapping simulated signal coverage of both the first simulated MIMOsignal and the second simulated MIMO signal at a third simulatedlocation in the simulated environment; and the at least one circuitfurther configured to analyze at least an imbalance of simulatedreceived power between the first simulated MIMO signal and the secondsimulated MIMO signal within the simulated environment at a thirdsimulated location in order to determine whether a desired capacity forsimulated MIMO communications with the simulated distributed antennasystem is achieved at the third simulated location in the simulatedenvironment.
 20. The apparatus of claim 19, wherein the at least onecircuit is further configured to: determine a simulated signal-to-noiseand interference ratio that is associated with at leas tone of the firstsimulated MIMO signal and the second simulated MIMO signal; determine asimulated channel condition number associated with the simulateddistributed antenna system from the analyzed simulated imbalance;determine a simulated capacity of the simulated distributed antennasystem from the signal-to-noise and interference ratio and the channelcondition number; and compare the simulated capacity for the simulateddistributed antenna system to a predetermined capacity threshold; anddetermine whether to increase or decrease the simulated imbalance inresponse to determining that the simulated imbalance is less than thepredetermined threshold.